Due to various advantageous characteristics of switching power converters, there have been developed many applications thereof, one of which is for LED drivers. In battery powered systems, for example LED flashlights, conventionally the input current is sensed and controlled in such a way that the lower the input power is the lower the input current is. Thus, LEDs can be lighted even when battery is almost exhausted and maximum utility time is realized. This method however has two major drawbacks: (1) In practice, the illumination of the LEDs is proportional to the output current instead of the input current, and thus the ‘wrong’ current is sensed and controlled; and (2) The input current is usually larger than the output current in a boost structure system, which makes this method not efficient, and considerable power is wasted on the current sense resistor.
For example, FIG. 1 shows a real application in which a boost structure LED driver 10 has a transistor Q acting as a power switch switched by a controller 12, and a current sense resistor R serially connected to the transistor Q for detecting the input current Iin to feed back to the controller 12. Once the voltage drop VR of the current sense resistor R is higher than a reference voltage, the transistor Q will be turned off for a constant time to release the energy stored in the inductor L. In this way, the peak current of the input current Iin is controlled. During the decrease of the input voltage Vin, the reference voltage drops at a constant slope to maximum the utility time of the battery. As mentioned above, the output current Io is not controlled and thus difficult to be determined. It will greatly change with external components and cause troubles for mass production. The efficiency is also hard to improve due to the large input current Iin flowing through the current sense resistor R.